With the Color tools you can manipulate image colors in several ways:
Modify the color balance: Sección 5.2, “Balance de color”
Adjust hue, saturation and lightness levels: Sección 5.3, “Tono-Saturación”
Render into a greyscale image seen through a colored glass: Sección 5.4, “Colorear”
Adjust brightness and contrast levels: Sección 5.5, “Brightness-Contrast”
Transform into a black and white image depending on pixel value: Sección 5.6, “Umbral”
Change the intensity range in a channel: Sección 5.7, “Niveles”
Change color, brightness, contrast or transparency in a sophisticated way: Sección 5.8, “Curvas”
Reduce the number of colors: Sección 5.9, “Posterizar”
Convert all colors to corresponding shades of gray: Sección 5.10, “Desaturate”